1. Field of the Invention
The present invention relates to strips or matrices of photosensitive elements
2. Description of the Prior Art
In order to construct matrices sensitive elements, it is a standard practice to provide an array of row leads and an array of column leads with a respective photosensitive element at each intersection of a row and a column. The row array is used to select a row of photosensitive elements whose electrical output signals are to be determined. The array of column leads is used to read a respective output signal for each element of the selected row.
One method for permitting individual selection of a row and individual reading on each column consists in employing at each point of the matrix the association of a photosensitive element proper and an access transistor. The photosensitive element is a photodiode or a phototransistor (this latter has the advantage of higher sensitivity or in other words of supplying a higher electric charge for the same illumination). This photodiode is connected by means of the access transistor to a column lead of the matrix. The transistor is controlled by a row lead (and all the photosensitive elements of the same row are thus connected to the same row conductor).
The disadvantage of this type of structure is that it involves the need to employ at each point of the matrix both a photodiode and an access transistor placed next to the photodiode. This structure is cumbersome on account of the area occupied by the access transistor at each point of the matrix.
In order to reduce the bulk and in fact in order to increase the resolution by housing a greater number of points in a given area, structures have been proposed in which the access transistor is suppressed and replaced by a capacitor which serves on the one hand to store the charges generated by the photodiode and on the other hand to isolate the photodiode from the column lead when the row corresponding to the photodiode is not selected.
A structure of this type is described for example in U.S. Pat. No. 4,797,560. The advantage of the capacitor with respect to the transistor is that it can be placed above and not next to the photodiode, thus reducing the bulk.
In particular, a practical method of construction of a matrix array of photosensitive elements consists in superposing three layers of p-doped, i-doped, and n-doped amorphous silicon and a dielectric layer, the entire assembly being sandwiched between a row lead and a column lead of the matrix.
Illumination of the photodiode takes place on one side or on the other and produces electric charges mainly in the intrinsic region of the diode. These charges accumulate at the interface between the photodiode and the capacitor or in other words within the n-type layer of the photodiode if the capacitor is located on the side corresponding to the n layer or within the p layer if this is not the case.
In accordance with the invention, it has been observed that it was possible in some cases to eliminate this doped layer entirely without substantially impairing the operation of the matrix except perhaps in regard to the speed of transfer of stored charges to the column lead through which reading of the signal generated by the photodiode is performed.
This possibility of suppression of the doped layer adjacent to the charged storage capacitor is based on the concept that a charge accumulation zone (of the same type as that of the doped layer) is formed at the interface between the photodiode and the capacitor as the illumination takes place.
In consequence, it is practically unnecessary to provide for veritable chemical doping of the interface in order to constitute a zone filled with free carriers.